Atmospheric Sciences & Global Change Research Highlights

August 2015

Demystifying Aerosols

PNNL researchers show that higher-resolution climate models can provide more accurate information on important particle-cloud interactions

Aerosol particle mysteries. PNNL researchers demonstrated that higher-resolution climate models can more accurately depict the important interactions between clouds and tiny aerosol particles from pollution, dust, and soot. The aerosols, not always as visible as in this view over a city, have an impact on clouds and climate. Image courtesy Wikimedia Commons License. Enlarge Image.

Results: How global climate models simulate something
so small as the interaction between tiny atmospheric particles and clouds can
have a surprisingly big impact on results. A research team led by Pacific
Northwest National Laboratory used innovative modeling and diagnostic methods
to demonstrate that higher-resolution models more accurately depict the
interactions between particles suspended in air, called aerosols, and the complex
processes within clouds. Going further, the team identified the physical
mechanisms behind the improved calculations. The results could guide the
development of next-generation climate models.

"Our analyses indicate that aerosol
effects on clouds change as the model detail increases," said Dr.
Po-Lun Ma, the PNNL atmospheric scientist who led the study reported in Geophysical Research Letters.

"However,
even in very high resolution models, there can be large differences between
model results and observations," said Ma. "Such differences suggest that resolution
increases are not enough, and the physical treatment of the aerosols and cloud
features requires further improvement to reduce the uncertainties associated
with aerosol-cloud interactions in climate models."

Why It Matters: Climate models involve millions of calculations on such things as air
flow, heat transfer, and movement of moisture through the atmosphere.
Researchers examine every process to ensure it functions as close to real-world
observations as possible so that results are reliable for understanding
climatic changes. Unfortunately, the exact mechanisms of interactions
between clouds and tiny aerosol particles from pollution, dust, and soot
remained largely a mystery. This study showed that higher resolution climate models
tend to more realistically depict how droplets form a cloud and how
precipitation develops. The results of such models more closely match estimates
based on real-world data.

Methods: Scientists used the Community Atmosphere Model version 5.2 (CAM5) to
zero in on aerosol-cloud interactions.
To facilitate direct comparison between model simulations and satellite
observations, they prescribed realistic meteorological fields such as winds and
temperature while allowing the model to use equations to calculate how clouds
and aerosols interact. They then examined four scenarios of varying degrees of
detail. The approach allowed them to compare model results with real-world data
at particular times and locations.

What's
Next? Scientists will
continue to work on ways to improve the way aerosol-cloud interactions are
portrayed in climate models. The PNNL team is currently applying the approach,
which grew out of the Aerosol Climate Initiative, to other types of simulations, so that future high-resolution climate
models will solve the mystery surrounding aerosol-cloud interactions.